\input pstfig
$
\thinline
\pnode(0,0){O}
\def\xmin{-3.5}	\def\xmax{3.5}
\def\ymin{-3}	\def\ymax{3}
\psaxes[labels=none,ticks=none,arrowlength=5,arrowinset=0.2,arrowsize=1.8pt]{->}(0,0)(\xmin,\ymin)(\xmax,\ymax)
\uput{0.2ex}[0](\xmax,0){x}
\uput{0.8ex}[-30](0,\ymax){y}
\uput[-135](O){O}
\thickline
\pnode(-2.8,2){A}
\pnode(A|O){B}
\pspolygon(A)(O)(B)
\thinline
\markang{->}(\xmax,0)(O)(A){0.8}
\uput{0.3}[70](O){\alpha}
\noneline
\pcline(O)(A)
\Bput{r}
\pcline(O)(B)
\Aput{x}
\pcline(A)(B)
\Bput{y}
\uput[55](A){P(x,y)}
$
\bye


